The present invention relates in general to electrophotography and, more specifically, to an electrophotographic imaging member and a process for forming the imaging member.
Multilayered photoreceptors have found increasing usage in electrophotographic copying machines and printers. The photoreceptors can be characterized as "layered photoreceptors" having at least a partially transparent photosensitive layer overlying a conductive ground plane. One problem inherent in using these layered photoreceptors becomes manifest when exposing the surface of the photoreceptor to a coherent beam of radiation, typically from a helium-neon or laser diode modulated by an image input signal. Depending upon the physical characteristics, two dominant reflections of the incident coherent light are on the surface of the photoreceptor; e.g., a first reflection from the top surface and a second reflection from the top surface of the relatively opaque conductive ground plane. This condition is shown in FIG. 1; coherent beams 1 and 2 are incident on a layered photoreceptor 6 comprising a charge transport layer 7, charge generator layer 8, and a ground plane 9. The two dominant reflections are: from the top surface of layer 7, and from the top surface of ground plane 9. Depending on the optical path difference as determined by the thickness and index of refraction of layer 7, beams 1 and 2 can interfere constructively or destructively when they combine to form beam 3. When the additional optical path traveled by beam 1 (dashed rays) is an integer multiple of the wavelength of the light, constructive interference occurs, more light is reflected from the top of charge transport layer 7 and, hence, less light is absorbed by charge generator layer 8. Conversely, a path difference producing destructive interference means less light is lost out of the layer and more absorption occurs within the charge generator layer 8. The difference in absorption in the charge generator layer 8, typically due to layer thickness variations within the charge transport layer 7, is equivalent to a spatial variation in exposure on the surface. This spatial exposure variation present in the image formed on the photoreceptor becomes manifest in the output copy derived from the exposed photoreceptor. FIG. 2 shows the areas of spatial exposure variation (at 25.times.) within a photoreceptor of the type shown in FIG. 1 when illuminated by a He-Ne laser with an output wavelength of 633 nm. The pattern of light and dark interference fringes look like the grains on a sheet of plywood. Hence the term "plywood effect" is generically applied to this problem.
One method of compensating for the plywood effect known to the prior art is to increase the thickness of and, hence, the absorption of the light by the charge generator layer. For most systems, this leads to unacceptable tradeoffs; for example, for a layered organic photoreceptor, an increase in dark decay characteristics and electrical cyclic instability may occur. Another method, disclosed in U.S. Pat. No. 4,618,552 is to use a photoconductive imaging member in which the ground plane, or an opaque conductive layer formed above or below the ground plane, is formed with a rough surface morphology to diffusely reflect the light. A still further method disclosed in co-pending application U.S.S. Ser. No. 07/523,639, assigned to the same assignee as the present invention, is to modify the imaging member by forming the ground plane itself of a low reflecting material.
A second problems associated with the layered photoreceptor is the possibility of separation (delamination) of one or more of the layers at one of the layered interfaces.
According to a first aspect of the present invention, the plywood effect is significantly reduced by suppressing the reflections from the conductive substrate. This is accomplished by coating the ground plane with a low-reflection coating of a material with a selected index of refraction, one preferred material being titanium oxide (T.sub.i O.sub.2). According to a second aspect of the invention, it has been found that a T.sub.i O.sub.2 layer in a preferred thickness range also greatly improves the adhesion of those layers vulnerable to delamination. More particularly, the invention relates to a photosensitive imaging member comprising at least a transparent photoconductive charge transport layer, overlying a charged generator layer and a conductive ground plane the ground plane being characterized by being coated with a low-reflection material having a refractive index greater than 2.05.